LEDs (Light Emitting Diodes) are sensitive to over voltage stress during manufacturing and in the field during operation. During manufacturing, LEDs are subject to electrostatic discharge (ESD) that may damage the LED. During operation the LEDs may also experience over voltage application to the LED. This over voltage stress may cause permanent damage. Consequently, the LEDs need ESD and overvoltage protection. This protection may be provided by a protection device.
Another problem occurs when LEDs are connected in a series configuration where a failure of one device shuts down the entire LED system. In such a situation, a protection device may work as a bypass, offering a low resistance current path parallel to the failed diode. Accordingly, the driving current is not blocked by the failed LED so that the remaining LEDs may continue to work.
Current LED protection devices may be Zener diodes and other discrete solutions. Usually a Zener diode protects one LED. When creating a LED bank with more than one diode in series the Zener diodes are placed parallel to each LED. If an over voltage event occurs, the Zener diode shunts current. But this configuration does not work as a bypass for an failed open LED because the voltage drop of the Zener diode in combination with the driving current causes to much heat.
An alternative discrete solution is to replace the Zener diode by an active circuit that offers a lower on resistance which allows creating a bypass when a LED fails. The main problem with all of these protection elements is a slow turn on time. A slow turn on time decreases the field of application for the LEDS and cannot be used for fast switching applications, such as for example, using a LED in a pulse width modulation (PWM) module.